Limits...
Floral miniaturisation and autogamy in boreal-arctic plants are epitomised by Iceland's most frequent orchid, Platanthera hyperborea.

Bateman RM, Sramkó G, Rudall PJ - PeerJ (2015)

Bottom Line: This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids.Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled.When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

View Article: PubMed Central - HTML - PubMed

Affiliation: Royal Botanic Gardens Kew , Richmond , United Kingdom.

ABSTRACT
Background and Aims. This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids. We draw particular attention to its phylogenetic placement, remarkable reproductive biology and morphological convergence on other Platanthera lineages through floral miniaturisation. Methods. Five populations of P. hyperborea in southwest Iceland were measured for 33 morphological characters and subjected to detailed multivariate and univariate analyses, supported by light and scanning electron microscopy of selected flowers. Representative samples from six populations were sequenced for nrITS and placed in a taxonomically broader phylogenetic matrix derived from previous studies. Key Results . Section Limnorchis consists of three distinct ITS-delimited clades based on P. stricta, P. sparsifolia-limosa-aquilonis and P. dilatata-hyperborea. Within the latter group, supposed species boundaries overlap; instead, the data indicate a crude stepwise series of ribotypic transitions extending eastward from North America to Iceland. Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled. Microscopic study of the flowers revealed several distinguishing features (some not previously reported), including resupinate lateral sepals, toothed bract margins, club-shaped papillae shared by both the interior of the labellar spur and the stigmatic surface, and an exceptionally adhesive stigma that is reliably covered in disaggregated pollen masses prior to anthesis; auricles are absent. Conclusions. Ribotypes suggest that Icelandic P. hyperborea represents the terminus of a migration route that may have begun in East Asia before passing through North America and presumably Greenland. The incohesive pollinia, rapidly desiccating anther locules, weakly developed rostellum, exceptionally adhesive stigma and the close juxtaposition of compact male and female reproductive organs together conspire to cause routine autogamy and frequent cleistogamy, despite the continued production of substantial nectar reservoirs in the spur and consequent ongoing attraction to the flowers of insects, including mosquitoes. When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

No MeSH data available.


Related in: MedlinePlus

Scenario suggesting how the two most phylogenetically divergent lineages within the genus Platanthera ultimately reached adjacent mid-Atlantic archipelagos by migrating (and evolving) circumboreally but in opposite directions.
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fig-16: Scenario suggesting how the two most phylogenetically divergent lineages within the genus Platanthera ultimately reached adjacent mid-Atlantic archipelagos by migrating (and evolving) circumboreally but in opposite directions.

Mentions: The evolutionary and migratory journeys undergone by the two lineages also bear comparison. Most molecular phylogenies of subtribe Orchidinae, including the most recent (Tang et al., 2015), strongly suggest that the subtribe originated in southeast Asia at 20 ± 9 Ma (Sramkó et al., 2014). Moreover, most of those topologies also suggest that the genus Platanthera s.l., and most of its eight monophyletic taxonomic Sections (cf. Hapeman & Inoue, 1997; Bateman et al., 2009), also originated in southeast Asia—the genus at 8 ± 3 Ma and the Sections a little later. Most of the Section-level lineages then migrated both eastward and westward, speciating as they migrated (Fig. 16). Based on present evidence, Section Platanthera has achieved its greatest species-level diversity in western Europe whereas Section Limnorchis has done so in North America, each lineage having expanded almost halfway around the globe but predominantly in opposite directions. The ‘final push’ into the Atlantic Ocean is hypothesised to have carried the Section Platanthera lineage ca 1,600 km westward from Iberia to the Azores, and the Section Limnorchis lineage ca 1,200 km eastward from southern Greenland to Iceland (Fig. 16). It is clear that the three Azorean species originated on the archipelago, whereas on present evidence it seems more likely that P. hyperborea originated before migrating to Iceland.


Floral miniaturisation and autogamy in boreal-arctic plants are epitomised by Iceland's most frequent orchid, Platanthera hyperborea.

Bateman RM, Sramkó G, Rudall PJ - PeerJ (2015)

Scenario suggesting how the two most phylogenetically divergent lineages within the genus Platanthera ultimately reached adjacent mid-Atlantic archipelagos by migrating (and evolving) circumboreally but in opposite directions.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4400879&req=5

fig-16: Scenario suggesting how the two most phylogenetically divergent lineages within the genus Platanthera ultimately reached adjacent mid-Atlantic archipelagos by migrating (and evolving) circumboreally but in opposite directions.
Mentions: The evolutionary and migratory journeys undergone by the two lineages also bear comparison. Most molecular phylogenies of subtribe Orchidinae, including the most recent (Tang et al., 2015), strongly suggest that the subtribe originated in southeast Asia at 20 ± 9 Ma (Sramkó et al., 2014). Moreover, most of those topologies also suggest that the genus Platanthera s.l., and most of its eight monophyletic taxonomic Sections (cf. Hapeman & Inoue, 1997; Bateman et al., 2009), also originated in southeast Asia—the genus at 8 ± 3 Ma and the Sections a little later. Most of the Section-level lineages then migrated both eastward and westward, speciating as they migrated (Fig. 16). Based on present evidence, Section Platanthera has achieved its greatest species-level diversity in western Europe whereas Section Limnorchis has done so in North America, each lineage having expanded almost halfway around the globe but predominantly in opposite directions. The ‘final push’ into the Atlantic Ocean is hypothesised to have carried the Section Platanthera lineage ca 1,600 km westward from Iberia to the Azores, and the Section Limnorchis lineage ca 1,200 km eastward from southern Greenland to Iceland (Fig. 16). It is clear that the three Azorean species originated on the archipelago, whereas on present evidence it seems more likely that P. hyperborea originated before migrating to Iceland.

Bottom Line: This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids.Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled.When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

View Article: PubMed Central - HTML - PubMed

Affiliation: Royal Botanic Gardens Kew , Richmond , United Kingdom.

ABSTRACT
Background and Aims. This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids. We draw particular attention to its phylogenetic placement, remarkable reproductive biology and morphological convergence on other Platanthera lineages through floral miniaturisation. Methods. Five populations of P. hyperborea in southwest Iceland were measured for 33 morphological characters and subjected to detailed multivariate and univariate analyses, supported by light and scanning electron microscopy of selected flowers. Representative samples from six populations were sequenced for nrITS and placed in a taxonomically broader phylogenetic matrix derived from previous studies. Key Results . Section Limnorchis consists of three distinct ITS-delimited clades based on P. stricta, P. sparsifolia-limosa-aquilonis and P. dilatata-hyperborea. Within the latter group, supposed species boundaries overlap; instead, the data indicate a crude stepwise series of ribotypic transitions extending eastward from North America to Iceland. Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled. Microscopic study of the flowers revealed several distinguishing features (some not previously reported), including resupinate lateral sepals, toothed bract margins, club-shaped papillae shared by both the interior of the labellar spur and the stigmatic surface, and an exceptionally adhesive stigma that is reliably covered in disaggregated pollen masses prior to anthesis; auricles are absent. Conclusions. Ribotypes suggest that Icelandic P. hyperborea represents the terminus of a migration route that may have begun in East Asia before passing through North America and presumably Greenland. The incohesive pollinia, rapidly desiccating anther locules, weakly developed rostellum, exceptionally adhesive stigma and the close juxtaposition of compact male and female reproductive organs together conspire to cause routine autogamy and frequent cleistogamy, despite the continued production of substantial nectar reservoirs in the spur and consequent ongoing attraction to the flowers of insects, including mosquitoes. When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

No MeSH data available.


Related in: MedlinePlus